Worcester Bosch Greenstar 35 CDi Fault codes & diagnostics

19 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 35 kW 2006-2012 Discontinued

Replaced by: Greenstar 37 CDi Compact

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
3 Emergency 12 High 1 Medium 3 Low
Just want the most common Worcester Bosch Greenstar 35 CDi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 19 documented codes

Tap any card for details

LED / flash-pattern codes
2 flashes

Your boiler is struggling to circulate water because there isn't enough pressure or there is air trapped in the system.

High DIY-safe £120-£350

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.
  2. If the pressure is too low, use the filling link underneath the boiler to top it up.
  3. Bleed your radiators to remove any trapped air that might be blocking water flow.
  4. Reset the boiler by holding the reset button for 3 to 5 seconds.

Parts commonly replaced

  • Circulating Pump
  • Expansion Vessel
  • Automatic Air Vent

Related codes

Full explanation for 2 flashes
3 flashes

Your boiler's temperature sensor for hot water has stopped working correctly, meaning your taps might not get warm.

Medium Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC sensor
  • Connecting wiring harness
Full explanation for 3 flashes
4 flashes

Your boiler has lost its memory chip, which tells it exactly how to operate safely, and it has stopped working as a safety precaution.

High Engineer only £120-280

Parts commonly replaced

  • HCM Code Plug
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 4 flashes
5 flashes

Your boiler's internal fan isn't spinning fast enough to safely clear away exhaust fumes, which has caused the system to shut down for safety.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Venturi tube

Related codes

Full explanation for 5 flashes
6 flashes

Your boiler's internal thermometer is broken, meaning it cannot accurately judge the temperature of the water to heat your radiators.

High Engineer only £90-160

Parts commonly replaced

  • Flow NTC thermistor
  • NTC wiring harness

Related codes

Full explanation for 6 flashes
7 flashes

Your boiler has sensed that the water inside is getting too hot and has shut itself down to prevent overheating.

High DIY-safe £90-180

What to check first

  1. Check that all radiator valves are fully open to allow water to circulate
  2. Ensure the system pressure is between 1 and 1.5 bar on the front gauge
  3. Reset the boiler by holding the reset button for five seconds

Parts commonly replaced

  • Overheat Thermostat
  • Central Heating Temperature Sensor (NTC)
  • Circulation Pump

Related codes

Full explanation for 7 flashes
8 flashes

Your boiler has tried to start but could not detect a flame, meaning it cannot heat your water or radiators and has shut down for safety.

High DIY-safe £120-£350

What to check first

  1. Check if other gas appliances in your home (like a hob) are working to ensure gas is reaching the property
  2. Check that your gas meter has credit if you have a pre-payment meter
  3. In freezing weather, check if your external condensate pipe is frozen and thaw it with warm water
  4. Press and hold the 'Reset' button for five seconds

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
  • PCB (Printed Circuit Board)

Related codes

Full explanation for 8 flashes
9 flashes

Your boiler’s internal computer has encountered a critical communication error and can no longer control the heating process safely.

High Engineer only £250-£550

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Internal Wiring Harness

Related codes

Full explanation for 9 flashes
Fast flash

Your boiler is still detecting a flame even after it has been told to turn off, which means it will shut down for safety purposes.

High Engineer only £120-350

Parts commonly replaced

  • Gas Valve
  • Ionisation Electrode
  • Control Board

Related codes

Full explanation for Fast flash
Slow flash

Your boiler detects a flame when it should be turned off, which is a safety concern that requires an immediate shutdown.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • Ionisation Probe
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Slow flash
Digital codes
-ll-

Your boiler is performing a routine self-check and filling its internal safety drainage system after the power was turned off and back on.

Low DIY-safe

What to check first

  1. Wait 15 minutes for the cycle to complete automatically
  2. Ensure the boiler power remains switched on
  3. The boiler will return to normal operation once the countdown finishes
Full explanation for -ll-
C6

Your boiler has stopped working because the internal fan that moves air and exhaust gases through the system isn't spinning fast enough.

High Engineer only £250-£450

Parts commonly replaced

  • Fan motor
  • PCB (Main Control Board)
  • Air-pressure switch
Full explanation for C6
d1

Your boiler is still detecting a flame even after it has been told to turn off, which may mean a valve is stuck or the sensors are faulty.

Emergency Engineer only £180-450

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Ionisation Lead
  • Electrodes

Related codes

Full explanation for d1
E9

Your boiler has sensed that it is getting too hot and has automatically shut down for safety.

High DIY-safe £120-£350

What to check first

  1. Check that your system pressure is between 1 and 1.5 bar on the pressure gauge
  2. Open all radiator valves fully to ensure water can move freely
  3. Press and hold the 'Reset' button for five seconds

Parts commonly replaced

  • Overheat Thermostat
  • Primary Heat Exchanger
  • Circulating Pump

Related codes

Full explanation for E9
EA

Your boiler is failing to light the gas flame or cannot detect that a flame is present, causing the system to shut down for safety.

High DIY-safe £100-350

What to check first

  1. Check if your gas supply is active by testing other gas appliances like a hob
  2. Ensure your gas meter has credit if you use a pre-payment meter
  3. Press and hold the 'Reset' button for five seconds to see if the system clears the fault

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for EA
F7

Your boiler thinks it still sees a flame even though the gas should be turned off, which is a safety conflict causing it to stop working.

High Engineer only £120-£350

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F7
FA

Your boiler thinks a flame is still present even though it has commanded the gas to turn off.

Emergency Engineer only £150-£450

Parts commonly replaced

  • Gas Valve
  • Ionisation Electrode
  • Printed Circuit Board (PCB)

Related codes

Full explanation for FA
No code

Your boiler is simply going through a self-test cycle after a manual reset and should return to normal operation within a few minutes.

Low DIY-safe

What to check first

  1. Wait 5-10 minutes for the boiler to complete its internal startup checks.
  2. Ensure your thermostat is turned up high enough to call for heat.
  3. Check if the display returns to showing a temperature or a blue light.
Full explanation for No code
Toggle on/off

Your boiler is simply flashing because the reset button was tapped, and it is waiting for a moment to restart its normal cycle.

Low DIY-safe

What to check first

  1. Wait 30 seconds for the boiler to complete its internal check
  2. If the light continues to flash, press and hold the reset button for five seconds
  3. Wait for the display to return to a steady temperature reading
Full explanation for Toggle on/off